Because of the high package density attainable with multilevel ceramic (MLC) substrate circuit structure, it has achieved extensive acceptance in the electronics industry for the packaging of semiconductor integrated devices, and other elements; as for example, see U.S. Pat. Nos. 3,379,943 granted Apr. 23, 1968 to J. D. Breedloff, 3,502,520 granted Mar. 24, 1970 to B. Schwartz, 4,080,414 granted Mar. 21, 1978 to L. C. Anderson et al, and 4,234,367 to L. W. Herron, et al granted Nov. 18, 1980.
In general, such conventional ceramic structures are formed from ceramic green sheets which are prepared from a ceramic slurry by mixing a ceramic particulate, a thermoplastic polymer and a solvent. This mixture is then cast or doctor bladed into ceramic sheets or slips from which the solvents are subsequently volatalized to provide a coherent and self-supporting flexible green sheet. This green ceramic sheet is laminated to form a substrate which is fired to drive off and/or burn off the binder and associated organic materials, and sintered to fuse the ceramic particulates together into a densified ceramic substrate.
In the fabrication of multilayer ceramic structures, an electrical conductor forming composition is deposited by spraying, dipping, screening, etc. in patterns on pre-punched green sheets, which when laminated and sintered collectively form the desired internal metallurgy structure. The component green ceramic sheets have via or feed through holes punched in them as required to achieve interconnection between layers or levels in the ultimate structure. The required number of component green sheets are stacked on each other in the required order. The stack of green sheets is then compressed or compacted at necessary temperatures and pressures to affect a bond between adjacent layers not separated by the electrical conductor forming patterns. The overall process is more completely described in "Ceramics For Packaging" by D. L. Wilcox, Solid State Technology, May 1972 P. 35-40.
Alumina, Al.sub.2 O.sub.3, because of its excellent insulating properties, thermal conductivity, stability and strength has received wide acceptance as the material of choice for fabrication of such MLC substrates. However, for various high performance applications, the relatively high dielectric constant of alumina results in significant signal propagation delays and noise. A further disadvantage of alumina is its relatively high thermal expansion coefficient, on the order of 65 to 70.times.10.sup.-7 per .degree.C., as compared to monocrystalline silicon where the coefficient is from 25 to 30.times.10.sup.-7, which difference may in certain cases result in some design and reliability concerns, particularly when a silicon chip is joined to the substrate with solder interconnections.
A particular disadvantage is the high sintering and maturing temperature of commercial alumina (about 1600.degree. C.), which restricts the choice of co-sinterable conductive metallurgies to refractory metals, such as tungsten, molybdenum, platinum, palladium, or a combination thereof. The high sintering temperature precludes the use of metals with higher electrical conductivities such as gold, silver or copper because the latter would be molten before the sintering temperature of alumina is reached.
A multilayer glass ceramic structure is disclosed and claimed in U.S. Pat. No. 4,301,324 by A. H. Kumar et al, whose teachings are incorporated herein by reference thereto, which avoids the use of and the attendant disadvantages of alumina ceramic. The disclosed multilayer glass-ceramic structures are characterized with low dielectric constants and are compatible with thick film circuitry of gold, silver, or copper and are co-fired therewith. Of the two types of glass-ceramics disclosed in the aforementioned patent, one has .beta.-Spodumene, Li.sub.2 O. Al.sub.2 O.sub.3. 4SiO.sub.2 as the principal crystalline phase while the other has cordierite, 2MgO. 2Al.sub.2 O.sub.3. 5SiO.sub.2 as the main crystalline phase. A common feature of these sintered glass-ceramics is their excellent sinterability and crystallization capability below 1,000.degree. C.
Silver can be used as a metal in such glass ceramic materials, however, it was found that silver has a tendency to cause electromigration problems and is suspected of diffusing into the glass ceramic.
Although successful glass-ceramic substrates have been made using gold metallurgy with a resistivity about 3.75 microhm-centimeter, gold is extremely expensive. Also, any alloying of other less expensive metals with gold would result in the disadvantage of an increase in resistivity. This limits the choice to copper as a practical economic alternative.
The use of copper is relatively new in the thick film technology. Because of copper's oxidizing potential, it is necessary to sinter multilayer ceramic substrates in reducing or neutral ambients. However, since reducing ambients can present adhesion problems, neutral ambients are preferable. A typical industrial cycle to sinter thick copper films on prefired alumina substrates would be heating to raise the temperature at the rate of 50.degree.-75.degree. C./minute to a firing or sintering temperature in the range of 900.degree.-950.degree. C. with a 15 minute hold at the peak temperature followed by cooling at a rate of 50.degree.-75.degree. C./minute.
In the fabrication of multilevel glass-ceramic structures, difficulty has been encountered in removing the resin binders that are used to produce the slurry for processing.
With the use of glass-ceramics and copper metallurgy, the maximum temperature for binder removal, due to the coalescence of glass particles, is in the range of about 800.degree.-875.degree. C. Thus, after the glass has coalesced, any remaining binder residue will become entrapped in the glass body. It has also been found that nitrogen or other neutral or reducing ambients make difficult the removal of binder below the temperature of the glass coalescence, e.g. about 800.degree.-875.degree. C., which can result in black or darkened substrates that are not fully sintered. The black or darkened color is generally attributed to carbon residue trapped in the ceramic. The carbon residue can have an adverse affect on the electrical characteristics of the resultant substrate, as by reducing the resistivity and having an adverse affect on the dielectric constant of the material.
Various efforts have been made to alleviate complete binder removal. Some difficulties are encountered with various neutral or reducing atmospheres, which include wet and dry nitrogen, wet and dry forming gas, long holds at temperature below the coalescence temperature of glass-ceramic so as not to entrap volatile products, and alternating air and forming gas for purposes of oxidizing carbon and reducing any form of copper-oxide to copper without drastic volume changes resulting from the copper oxide formation. In addition, attempts have been made to develop a polymer binder system which would by some mechanism of fractionation (e.g. unzipping, hydrolosis, etc.) burn off without any remaining carbonaceous residue.
U.S. Pat. No. 4,234,367 by Herron et al, issued Nov. 18, 1980 discloses and claims a method of forming a glass-ceramic composite structure with a copper-based metallurgy. In this process, laminated green ceramic substrates are heated in a H.sub.2 /H.sub.2 O environment to a burn-out temperature in the range between the anneal and softening point of the glass-ceramic material which temperature is less than the melting point of the copper. The conditions are sufficient to remove the binder without oxidizing the copper metallurgy. The resultant binder-free laminate is then heated in a nitrogen atmosphere to a somewhat higher temperature to coalesce the glass particles. The copper is prevented from oxidizing in this later heating phase by the inert atmosphere.
While the process is operable and effective, the binder removal heating step is quite lengthy, and carbonaceous residues may remain in the substrate. Further, the ceramic portions about the copper metallurgy may be porous. This flaw is very serious if it occurs under a surface pad which is later stressed as by bonding to a pin or other interconnection, resulting in ceramic fail due to low strength.